B − V colour curves of 12 SNe for which an Na iD absorption line is detected, dereddened according to various relations from the literature; a decrease in scatter is observed only in (b), where the average σ is reduced by 9 per cent, whereas (c) and (d) show an increase of 40 and 15 per cent (respectively) in scatter. The shaded area represents ±1σ from the mean colour.